Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, and Huntington's disease occur as a result of neurodegenerative processes. Neurodegeneration can be found in many different levels of neuronal circuitry ranging from molecular to systemic.
Alzheimer's disease (AD) is characterised by loss of neurons and synapses in the cerebral cortex and certain subcortical regions. AD individuals show 70% loss of locus coeruleus cells that provide norepinephrine. Norepinephrine, in addition to its neurotransmitter role, locally diffuses from “varicosities” as an endogenous antiinflammatory agent in the microenvironment around the neurons, glial cells, and blood vessels in the neocortex and hippocampus. It has been shown that norepinephrine stimulates mouse microglia to suppress β-amyloid (Aβ)-induced production of cytokines and their phagocytosis of Aβ. It is estimated that about 18 million people worldwide have AD.